1. Field of the Invention
The present invention relates to the field of detection apparatus used to screen individuals for the presence of explosives and other chemical entities.
2. Background Information
The growing concern over security in public venues has led to development of new technologies capable of rapidly detecting the presence of illicit and hazardous materials. One of the lead organizations in this effort is the Transportation Security Agency (TSA), who is responsible for ensuring safety of air travel and has invested significantly in developing technologies to combat the potential for attacks by explosive devices.
Two types of detectors are used for screening baggage and people. Explosive detection systems (EDSs) detect bulk explosives hidden in checked baggage and frequently operate using dual x-ray tomography. Explosives trace detectors (ETDs) detect vapor or particles of explosives that are contaminated on people and the surface of baggage. ETDs are also used to resolve alarms from EDSs. Currently ETDs are used on a selective basis to screen for personal items and carry-on bags, but not for directly screening individuals. The lack of a capability to screen for explosives hidden on an individual is arguably the greatest vulnerability in aviation security. Though most of the attention for explosives threat detection is focused on aviation security, in fact security is an issue for many venues including other types of transportation, buildings, ports, stadiums, military base and field operations, and in general any high traffic environment.
Several portal concepts using ETDs have been developed and tested. The most promising are based on non-intrusive (non-contact) removal of particles from clothing, followed by high-flow collection of the particles on the surface of a mesh or substrate, and then thermal desorption into an ETD. For examples Linker et al. disclosed in U.S. Pat. No. 5,915,268 a portal device that uses air jets to dislodge particles from a person and a downward flow of air to entrain and carry the particles to a concentrator device. Another portal method was disclosed by Settles in U.S. Pat. No. 6,073,499, which involves a passive method of particle collection relying on the upward flow of air around people due to thermal conductivity in what is called the human thermal plume (HTP). Jenkins et al disclosed in U.S. Pat. No. 6,708,572 a similar method that uses air jets to assist the upward flow of the HTP. Each of these portal methods involves a flow of particle and vapor laden air and uses a concentration device that removes the target particles and vapor from the large volume of air by collecting them onto a mesh or substrate. The target particles and vapor are then thermally desorbed and mixed with a low volume flow of gas that leads to a chemical analyzer. Another portal method was disclosed by Bromberg et al in U.S. Pat. No. 5,760,314 and, which unlike the above non-contact portals, uses samplings tubes that make contact with individuals to efficiently collect particles and vapor.
A two-stage concentrator for vapor/particle detection was disclosed by Linker and Brusseau in U.S. Pat. No. 6,345,545. Their device enables concentrating particles from a high-volume gas flow to a sufficiently low-volume gas flow to be coupled efficiently to a detector. The first stage of the concentrator contains a metal mesh for collecting particles and vapor that are entrained in the high-volume gas flow. The desorbed vapors from the first-stage concentrator are collected on the second-stage concentrator and provided to the detectors with a low-volume gas flow.
The chemical detectors used in all of the portals mentioned above use some form of ETDs including ion mobility spectrometry (IMS), mass spectrometry (MS), and gas chromatography/chemiluminescence (GC/CL) detectors.
An important characteristic of a personnel screening portal is that it operate sufficiently fast so as not to significantly impede the flow of people going through it. Current portals operate with a sampling interval of about 10-15 s from one person to another. This is much longer than the interval of about 4-6 s for which standard metal detectors are capable. Methods to improve the sampling interval of a vapor/particle detecting portal are highly desirable.
Another important characteristic of a portal is that it have reasonable upfront and recurring costs per unit. Given the large number of passenger lanes in U.S. and international airports (about 3000 each) and limited available budgets, inevitable compromises are made with regard to cost and the number of units that can be deployed. Solutions that reduce the cost per passenger screened would allow greater distribution of deployed screening portals.